1. Field of the Invention
This invention relates to special effects and, more particularly, to the special effects obtainable by combining a plurality of image information signals into a keyed, true-transparency image information signal combine.
2. Description of Related Art
Various arrangements exist in the art for selectively changing, combining or manipulating one or more image information signals such as video image signals to achieve special effects such as might be viewed in a television system.
For example, U.S. Pat. No. 3,598,908, which is entitled "Digitally Controlled Lap Dissolver" and which issued Aug. 10, 1971, discloses a dissolver network for combining two video signals into a composite video signal. The dissolver network includes an attenuator network that incrementally changes the signal level of a first video input signal V.sub.1 from a first level limit and concurrently changes the signal level of a second video input signal V.sub.2 from a second level limit. The attenuator network operates at a clock rate corresponding tothe video field rate, for example, at a rate of one times, or even at a sub-multiple of, the vertical synchronization rate. That scheme provides a uniform dissolve operation wherein the image information in the first video signal V.sub.1 is eventually replaced by the image information in the second video signal V.sub.2, and vice-versa. However, during the process of replacing the first signal with the second signal, the first and the second video signals are proportionately combined to form a composite video signal V.sub.C, which has the property that the summation of the varying proportions k.sub.1 and k.sub.2, respectively, for the first and the second video signals V.sub.1 and V.sub.2, respectively, remains at one hundred percent over the dissolve time interval. The foregoing words can be restated using standard mathematical notation as: EQU V.sub.C =k.sub.1 V.sub.1 +k.sub.2 V.sub.2 ( 1)
where k.sub.1 +k.sub.2 =1.0, which can be rewritten as: EQU V.sub.C =k.sub.1 V.sub.1 +(1-k.sub.1)V.sub.2.
The visual result from the mixing algorithm of equation (1) convincingly displays the first and the second video signals V.sub.1 and V.sub.2 as video images that may be transparent. That is to say, as V.sub.1 dissolves into V.sub.2, the visual result to the viewer appears as though an originally opaque V.sub.1 becomes gradually more transparent, revealing V.sub.2 as though V.sub.2 were behind V.sub.1.
Known switcher technology makes use of this mixing algorithm in a cascaded form in order to create a special video effect called a combine. A "combine" is a special effect obtainable by combining image information signals from two or more channels, for example, by combining N channels of video signals V.sub.i together with a generated background video signal V.sub.B in such a way as to make the scenes or images corresponding to the video signals appear to the viewer to be spatially related to each other in some manner and in accordance with some defined priority. Thereby, a combiner may create the special effect of a first video image from a first channel of higher priority being in front of a second video image from a second channel of lower priority in the sense that the first video image is seen to be closer to the viewer than is the second video image. Hence, a rank ordering, also called a prioritized ordering, can be defined among a plurality of video images such that, for example, a first video image from a first channel is seen to be in front of a second video image from a second channel, which, in turn, is seen to be in front of a third video image from a third channel, etc.
This special effect is created by cascading two or more mix/effect (M/E) units contained within the switcher, each of which is capable of embodying the mixing algorithm of equation (1). In so doing, the name "true transparency" algorithm is given to the mixing algorithm. For example, assume that it is desired to mix a first video signal V.sub.1 over a second video signal V.sub.2 over a third video signal V.sub.3 over a background signal V.sub.B. This can be implemented, for example, by using a key signal k.sub.3 to mix the third video signal V.sub.3 over the background signal V.sub.B to obtain a first mixed effect M/E.sub.1 of: EQU M/E.sub.1 =V.sub.3 k.sub.3 +V.sub.B (1-k.sub.3).
In turn, by using a key signal k.sub.2 to mix the second video signal V.sub.2 over the first mixed effect M/E.sub.1, one obtains a second mixed effect M/E.sub.2 of: ##EQU1## Still further in turn, by using a key signal k.sub.1 to mix the first video signal V.sub.1 over the second mixed effect M/E.sub.2, one obtains a third mixed effect M/E.sub.3 of: ##EQU2## The true transparency algorithm is further described in the article by Loren Carpenter, "The A-buffer, an Antialiased Hidden Surface Method," Computer Graphics, Vol. 18, No. 3 (Jul. 1984), pp. 103-108.
By using the foregoing methodology of cascading the true transparency algorithm, each video image can be defined to be transparent by its corresponding key signal, and a convincing display of multiple transparent video images is still obtained. Notice, however, that in order to change the priority of the video images, the video images themselves are switched among the various M/E units. This architecture can thus become cumbersome when processing digital video signals, making it desirable to consider an alternative architecture.
As an example, U.S. Pat. No. 4,758,892, which is entitled "System for Producing a Video Combine from Multiple Video Images" and which issued Jul. 19, 1988 and which is incorporated herein by reference, discloses an alternative architecture for creating a combine. That alternative system generates a display of prioritized video images, but it does so without switching the video signals themselves. Rather, the video processing path for each video signal is dedicated as is a corresponding key processing path for each respective key signal. Instead of switching cumbersome video signals, the prior art alternative accomplishes similar results by switching only "key-taken" signals. The channel signals to be combined are synchronized so that digitally encoded video image signals, which define the respective video images in the respective channels, arrive at processing logic situated in the combiner during the same clock cycle for processing. Thereby, the video images are combined while being processed by the combiner to form the combined video signal V.sub.C. The combining process involves taking a respective preselected portion k.sub.i of the value of each respective digitally encoded video image signal V.sub.i and adding the products thereby formed in such a manner that the total of the respective portions including any background portion k.sub.B, as a percentage, is equal to one hundred percent. The foregoing words can be restated using standard mathematical notation as: EQU V.sub.C =k.sub.1 V.sub.1 +k.sub.2 V.sub.2 +. . . +k.sub.N V.sub.N +k.sub.B V.sub.B ( 2)
where k.sub.1 +k.sub.2 +. . . +k.sub.N +k.sub.B =1.0 and where N is the number of video channels, whose signals V.sub.i are combined in respective portions k.sub.i, and where k.sub.B is the portion of any background video signal V.sub.B that is included in the combine.
As an aside, it can be mentioned that the combiner process of taking a selected portion, or percentage, of the value of a typical digitally encoded image information signal can be accomplished by multiplying the value of the selected portion and the value of the image signal. That process is called "cutting" in the art and, in that manner, a video signal is said to be "cut".
It should be noted, however, that the particular mix algorithm described in the prior art patent does not result in the display of video images, which are convincingly transparent. One remaining problem is that lower priority video images such as those that are more distant from a viewer, when viewed through one or more transparent higher priority video images such as those that are closer to the viewer, may dominate the combined video signal in the sense that there is created an undesirable effect, which is manifested, in part, by the perception that some of the more distant lower priority video images appear to be closer to the viewer and to unnaturally dominate the combine than do some of the closer higher priority images.
Therefore, while the combiner architecture is preferable to that of conventional switcher technology when processing digital video signals, the mix algorithm embodied in the combiner yields undesirable results.